Add table of contents to the right side of pages in the docs

[logankilpatrick]

See this issue for details: JuliaDocs/Documenter.jl#1688 but TLDR; we waste a lot of usable space on the right side of the docs which could be used for really neat stuff like is done in the VS Code docs: https://code.visualstudio.com/docs

[rmNULL]

Description

Hi, newcomer to julia here. I'm having trouble navigating the documentation.

If i want to find the name of a particular function, i have to manually scroll through all the documentation of that particular section.
Having an overview of what functions are available under a particular section would have been helpful.
Base Strings is one of the page i was looking at.

Examples from other languages

I'll leave links to documentation of other languages as to what i meant by overview.

Ruby

Racket

Some clarifications.

I don't mean to say we have to take it as it is from other languages. Having overview of some kind will be beneficial. I haven't opened a new issue as these issues are related.
An alternate solution i see to this problem is to have an expandable Card which hides its content by default.

what am i going to do about it?

I'm willing to contribute to improve the existing documentation.
but i wasn't sure if opening a PR with these changes would have been a good outcome as i'm not sure whats the right way to tackle this issue.

Thanks.

[logankilpatrick]

@rmNULL you would want to take a look at: JuliaDocs/Documenter.jl#1688 and potentially get involved there.